System for assisting regeneration of a storage/releasing nox trap for a motor vehicle diesel engine

ABSTRACT

A system for assisting regeneration of a storage/release NOx trap integrated in an exhaust line ( 5 ) of a motor vehicle diesel engine ( 4 ) is characterized in that it comprises gas admission means for admitting gas into the engine, means for injecting fuel ( 11 ) into the cylinders thereof in the form of at least pilot and main injections, and means for controlling ( 12 ) said gas admission and/or fuel injection means for periodically switching the engine ( 4 ) between a lean-mixture standard operating mode in which NOx is stored in the trap ( 6 ) and a rich-mixture regeneration operating mode, with at least two pilot injections ( 1, 2 ) and one main injection ( 3 ), in which NOx is released from the trap ( 6 ) and the trap is regenerated.

The present invention relates to a system for reducing pollutantemissions associated with the operation of a motor vehicle dieselengine, and more particularly to a system for assisting regeneration ofa storage/release NOx trap integrated into an engine exhaust line.

With direct injection diesel engines for motor vehicles, reducing NOxemissions by storage/release DeNOx catalysis is one technical solutionenvisaged for compliance with the standards relating to this type ofpollution, and in particular the EURO IV standards.

To this end, it is envisaged to use a NOx trap adapted to absorb NOxduring standard operation of the engine and consisting of bariumsulfate, for example.

It is proposed to trigger a transient release stage when the trap issaturated by momentarily switching the engine from the standard mode ofoperation with a lean mixture to a regeneration mode of operation with arich mixture in order to produce reducing agents, for example HC and CO,that reduce the NOx, which is then desorbed from the trap, as in aconventional catalytic converter.

However, that causes problems with stable control of the operation ofthe engine when using a rich mixture over the whole of its range ofoperation without impacting on driver comfort, combustion noise, etc.

The object of the invention is therefore to solve the above problems.

To this end, the invention consists in a system for assistingregeneration of a storage/release NOx trap integrated in an exhaust lineof a motor vehicle diesel engine, the system being characterized in thatit comprises gas admission means for admitting gas into the engine,means for injecting fuel into the cylinders thereof in the form of atleast pilot and main injections, and means for controlling said gasadmission and/or fuel injection means for periodically switching theengine between a lean-mixture standard operating mode in which NOx isstored in the trap and a rich-mixture regeneration operating mode, withat least two pilot injections and one main injection, in which NOx isreleased from the trap and the trap is regenerated.

According to other features of the invention:

-   -   the control means are adapted to control the gas admission means        to reduce the quantity of gas admitted into the engine when said        engine is in its regeneration mode of operation;    -   the control means are adapted to control the gas admission means        and/or the fuel injection means in accordance with the standard        and regeneration modes of operation for engine loads below a        predetermined threshold value;    -   the predetermined load threshold value is defined by a brake        mean effective pressure (bmep) of approximately 3 bars;    -   the engine is associated with exhaust gas recirculation means        for recirculating exhaust gas to its inlet, and the control        means are adapted to regulate the operation of the recirculation        means during operation of the engine with a rich mixture;    -   the two pilot injections are triggered in a crankshaft angle        range from approximately 50° to approximately 5° ahead of the        top dead centre point of the cylinder concerned and the main        injection is triggered in an undercalibrated range up to a        crankshaft angle of approximately 35° after the top dead centre        point;    -   the control means are adapted to control the gas admission means        and/or the injection means to operate the engine with a lean        mixture for approximately 60 seconds and a rich mixture for        approximately 2 seconds.

The invention will be better understood on reading the followingdescription, which is given by way of example only and with reference tothe appended drawings, in which:

FIGS. 1 and 2 show the NOx storage and release stages in a trap includedin a system of the invention;

FIG. 3 shows the operation of fuel injection means used in a system ofthe invention; and

FIG. 4 is a block diagram of the structure of a system of the invention.

As indicated above, the invention relates to a system for assistingregeneration of a storage/release NOx trap integrated in an exhaust lineof a motor vehicle diesel engine.

FIGS. 1 and 2 show the storage and release stages.

As also indicated, the NOx trap absorbs NOx during standard operation ofthe engine with a lean mixture, whereas in a regeneration mode ofoperation with a rich mixture NOx is released and treated by producingreducing agents such as HC and CO, that reduce NOx, which is thendesorbed from the trap as in a conventional catalytic converter.

However, and as indicated above, this leads to the problem of stableoperation of a diesel engine when using a rich mixture over the whole ofthe engine range without impacting on driver comfort, combustion noise,etc.

Switching to a stoichiometric or rich mixture is necessary for purgingthe NOx trap, and also for desulfating it, as sulfur in the fuel poisonsthe trap.

This operation takes longer than purging, which takes only a fewseconds, and is therefore even more difficult to manage.

Under such circumstances, assisting the regeneration of astorage/release NOx trap necessitates the development of a specificmultiple injection strategy applicable over the whole of the enginerange and based on a plurality of engine parameters.

The injection strategy must:

-   -   enable operation at a richness close to 1;    -   produce CO in sufficient quantities, since this is the preferred        reducing agent;    -   limit the level of O₂;    -   limit the level of HC;    -   conform to smoke level criteria;    -   conform to exhaust gas temperature criteria for reasons of        mechanical strength; and, finally    -   conform to criteria in respect of services provided to end        users, namely driver comfort and engine operating noise.

To this end, the present invention proposes to improve the stability andnoise characteristics of combustion at predetermined load points of theengine by applying an injection strategy employing two pilot injectionsand one main injection, as shown in FIG. 3.

Applying the two pilot injections retards the main injection to satisfythe criteria of NOx trap regeneration.

Moreover, exhaust gas recirculation (EGR) can be regulated to conform tothe NOx trap regeneration criteria over all these points.

At the predetermined load points, for example for a brake mean effectivepressure (bmep) less than about 3 bars, the strategy employing two pilotinjections therefore satisfies the NOx trap regeneration criteria, sinceit significantly reduces combustion instabilities at those points,because of the phasing of the two pilot injections, reduces noise, alsobecause of the phasing of the two pilot injections, retards the maininjection, and finally generates an engine map for regeneration of theNOx trap based on a unique strategy.

As indicated above, that strategy may be applied at predeterminedoperating load points of the engine, to regulate EGR.

This is represented in FIG. 3, which shows the two pilot injections 1and 2 and the main injection 3.

The two pilot injections can be triggered in a crankshaft angle rangefrom approximately 50° to approximately 5° ahead of the top dead centrepoint of the cylinder concerned, and the main injection can be triggeredin an undercalibrated range up to a crankshaft angle of approximately35° after the top dead centre point.

FIG. 4 shows a motor vehicle diesel engine 4 whose outlet is connectedto an exhaust line 5 in which a NOx trap 6 is integrated.

The engine is also associated with air admission means 7 including means8 for adjusting the quantity of air admitted.

The engine is also associated with means 9 for recirculating exhaustgases to its inlet associated with means 10 for controlling thecirculation of the gas.

The exhaust gas recirculation (EGR) means are therefore connectedbetween the outlet and the inlet of the engine to recirculate theexhaust gases.

The adjustment means 8 and the control means 10 can comprise valves inthe conventional way.

The engine is also associated with means 11 for injecting fuel into itscylinders in the form of pilot injections and main injections, asalready indicated, such means having any appropriate structure.

The various means, i.e. the exhaust gas recirculation means, the airadmission means and the injection means, are controlled by control means12 comprising any appropriate computer receiving as input, for example,engine rotation speed information and injection pressure informationfrom corresponding data acquisition means 13 and 14, respectively.

The control means 11 may periodically switch between the standardoperating mode with a lean mixture that stores NOx in the trap and theregeneration mode of operation with a rich mixture that releases NOxfrom the trap and thereby regenerates the trap, the engine 4 operatingwith a lean mixture for approximately 60 seconds and a rich mixture forapproximately 2 seconds, for example. The control means 12 then operateon the air admission means 7 for admitting air into the engine 4 and theinjection means 11 for injecting fuel into its cylinders, to switch theengine periodically from the lean-mixture standard mode of operation tothe rich-mixture regeneration mode of operation.

The control means 12 may be adapted to control the gas admission meansin order to reduce the quantity of gas admitted into the engine when itis in its regeneration operating mode and to control the injection meansin the manner indicated above.

The control means can also be adapted to regulate the operation of theexhaust gas recirculation means 9 when the engine is operating with therich mixture.

Other embodiments can be envisaged, of course.

1. A system for assisting regeneration of a storage/release NOx trapintegrated in an exhaust line of a motor vehicle diesel engine, thesystem comprising gas admission means for admitting gas into the engine,means for injecting fuel into the cylinders thereof in the form of atleast pilot and main injections, and means for controlling said gasadmission and/or fuel injection means for periodically switching theengine between a lean mixture standard operating mode in which NOx isstored in the trap and a rich mixture regeneration operating mode inwhich NOx is released from the trap and the trap is regenerated, whereinin a rich-mixture regeneration operating mode, the injection means aresuitable for implementing at least two pilot injections triggered in acrankshaft angle range from approximately 50° to approximately 5° aheadof the top dead centre point of the cylinder concerned and the maininjection is triggered in an undercalibrated range up to a crankshaftangle of approximately 35° after the top dead centre point.
 2. A systemaccording to claim 1, wherein the control means are adapted to controlthe gas admission means to reduce the quantity of gas admitted into theengine when said engine is in its regeneration mode of operation.
 3. Asystem according to claim 1, wherein the control means are adapted tocontrol the gas admission means and/or the fuel injection means inaccordance with the standard and regeneration modes of operation forengine loads below a predetermined threshold value.
 4. A systemaccording to claim 3, wherein the predetermined load threshold value isdefined by a brake mean effective pressure of approximately 3 bars.
 5. Asystem according to claim 1, wherein the engine is associated withexhaust gas recirculation means for recirculating exhaust gas to itsinlet, and the control means are adapted to regulate the operation ofthe recirculation means during operation of the engine with a richmixture.
 6. (canceled)
 7. A system according to claim 1, wherein thecontrol means are adapted to control the gas admission means and/or theinjection means to operate the engine with a lean mixture forapproximately 60 seconds and a rich mixture for approximately 2 seconds.